In electric speedometers, a transducer is associated with the speedometer cable drive gear in the transmission housing to produce an input signal by a series of pulses from the wheel or drive shaft. A visual indication of the speed and distance traveled from the periodic signal will be visualized since speed and distance traveled are a linear function in regard to the incoming frequency.
Tachometers sense the frequency of pulses from the alternator or magnetic pickup. A visual indication of the cycling rate is provided as a linear function proportional to the incoming frequency.
The problem with such electric speedometers and tachometers is that a change in the gear ratio or tire size of the wheels with regard to speedometers and a change in pulley or belt size with regard to tachometers will alter the calibration of the respective meter rendering them inaccurate until recalibrated. The vehicle manufacturer must stock a different speedometer unit for each tire size and/or axle ratio and a different tachometer for each pulley size and/or belt size. Adjustments may be made within the speedometer and tachometer units themselves, but require precision adjustment and calibration. Moreover, a separate adjustment of the odometer range may be necessary. The adjustments require sophisticated operator training, tools, and calibration equipment not normally available outside of the factory. Also, other systems employing adjusting mechanisms are relatively complex and costly.